Ion implantation has been a critical technology in semiconductor device manufacturing and is currently used for many processes including fabrication of the p-n junctions in transistors, particularly for CMOS devices such as memory and logic chips. By creating positively-charged ions containing the dopant elements required for fabricating the transistors in silicon substrates, the ion implanters can selectively control both the energy (hence implantation depth) and ion current (hence dose) introduced into the transistor structures. Traditionally, ion implanters have used ion sources that generate a ribbon beam of up to about 50 mm in length. The beam is transported to the substrate and the required dose and dose uniformity are accomplished by electromagnetic scanning of the ribbon across the substrate, mechanical scanning of the substrate across the beam, or both. In some cases, an initial ribbon beam can be expanded to an elongated ribbon beam by dispersing it along a longitudinal axis. In some cases, a beam can even assume an elliptical or round profile.
Currently, there is an interest in the industry in extending the design of conventional ion implanters to produce a ribbon beam of larger extent. This industry interest in extended ribbon beam implantation is generated by the recent industry-wide move to larger substrates, such as 450 mm-diameter silicon wafers. During implantation, a substrate can be scanned across an extended ribbon beam while the beam remains stationary. An extended ribbon beam enables higher dose rates because the resulting higher ion current can be transported through the implanter beam line due to reduced space charge blowup of the extended ribbon beam. To achieve uniformity in the dose implanted across the substrate, the ion density in the ribbon beam needs to be fairly uniform relative to a longitudinal axis extending along its long dimension. However, such uniformity is difficult to achieve in practice.
In some beam implanters, corrector optics have been incorporated into the beam line to alter the ion density profile of the ion beam during beam transport. For example, Bernas-type ion sources have been used to produce an ion beam of between 50 mm to 100 mm long, which is then expanded to the desired ribbon dimension and collimated by ion optics to produce a beam longer than the substrate to be implanted. Using corrector optics is generally not sufficient to create good beam uniformity if the beam is greatly non-uniform upon extraction from the ion source or if aberrations are induced by space-charge loading and/or beam transport optics.
In some beam implanter designs, a large-volume ion source is used that includes multiple cathodes aligned along the longitudinal axis of the arc slit, such that emission from each cathode can be adjusted to modify the ion density profile within the ion source. Multiple gas introduction lines are distributed along the long axis of the source to promote better uniformity of the ion density profile. These features attempt to produce a uniform profile during beam extraction while limiting the use of beam profile-correcting optics. Notwithstanding these efforts, the problem of establishing a uniform ion density profile in the extracted ion beam remains one of great concern to manufacturers of ribbon beam ion implanters, especially when utilizing ion sources having extraction apertures dimensioned in excess of 100 mm. Therefore, there is a need for an improved ion source design capable of producing a relatively uniform extracted ion beam profile.